The invention relates to variable speed belt driven transmission systems, but more particularly, the invention relates to variable speed belt driven transmission systems apparatus, and methods for improving axial forces at the driven pulley of the transmission for enhanced belt life.
Adjustable speed V-belt drives are variable speed belt transmission systems which are either manually or automatically regulated. Belt driven transmissions are used in various machinery such as agricultural equipment, snowmobiles, automobiles, and industrial equipment. The drives are powered at some peak torque by some source such as a motor, and may be required to deliver power at various speed ratios and torques to a constantly changing output load. In automotive applications, for example, an internal combustion engine having peak and transient torque characteristics, delivers power at various speed ratios through a transmission to vehicle wheels that react to changing road loads (e.g., windage, hills, and speed). Belt driven transmissions customarily are designed to automatically shift to accommodate changing road loads.
The prior art is replete with examples of automatic pulley shifters or actuators that are either speed responsive, torque responsive, or combinations thereof. The shifters may be mechanically operated, electrically operated, pneumatically or hydraulically operated. The speed responsive system may use centrifugal fly weights, and torque responsive actuators may use helical torque ramps or a hydraulic pressure that is generally related to torque. This invention is primarily directed to a belt driven transmission system which uses a torque sensing device in association with the driven pulley of the belt driven transmission.
In comparison to the replete examples of pulley actuators, there are only a handful of references dealing with mathematical analysis of the various kinds of variable speed belt transmissions. However, the references are excellent and some of them are: (1) W. S. Worley, "Designing Adjustable-Speed V-Belt Drives For Farm Implements." SAE Transactions, Vol. 63 (1955); (2) L. R. Oliver and V. D. Henderson, "Torque Sensing Variable Speed V-Belt Drive." SAE Transactions, Vol. 81 (1972); (3) B. G. Gerbert, "Force and Slip Behavior and V-Belt Drives." Acta Polytechnica Scandinavica, MECH. ANG. Series No. 67, Helsinki, (1972); (4) B. G. Gerbert, "Adjustable Speed V-Belt Drives-Mechanical Properties and Design." SAE Paper 740747 (1974); (5) B. G. Gerbert, Doctors Thesis on V-Belt Drives with special reference to force conditions, slip, and power loss." Lund Technical University, Lund, Sweden, (1973); and (6) B. G. Gerbert, "A Complementary Large Slip Solution In V-Belt Mechanics," ASME Paper 77-DET-162 (1977).
Reference (4) Supra, analyzes various types of adjustable speed V-Belt drives and at page 5, example 5, a driven pulley with a torque ramp for closing the pulley halves together responsive to rotational torque, is discussed in conjunction with FIG. 3 showing driven pulley axial forces as a function of the coefficient of traction. The axial force coefficient of traction type charts are useful for showing an axial force, tension interrelationship for a variable speed belt drive. Dimensionless axial force, F/(T.sub.1 +T.sub.2) where F is axial force, T.sub.1 is tight side belt tension and T.sub.2 is slack side belt tension, is scaled on the ordinate and traction ratio (T.sub.1 -T.sub.2)/(T.sub.1 +T.sub.2), is scaled on the abcissa. Such charts show that dimensionless axial force at the driven pulley is generally a constant band for all traction ratios and speed ratios. Comparatively, the dimensionless axial force at the driver pulley drastically increases with traction ratio for all speed ratios. Thus, the axial forces at the driven pulley generally define the total tension (T.sub.1 +T.sub.2) in the drive as well as the force available to produce torque (T.sub.1 -T.sub.2) for transmitting power. Of course, the interrelationship between axial force and traction ratio is influenced by belt design, pulley diameter, and pulley center distance. These interrelationships are also discussed in the above mentioned references.
For the purpose of this disclosure, the characteristics of the surface contact between the sidewalls of a V-belt and the surfaces of a pulley are described as having one of two possible extreme conditions:
1. "Slip" is defined as a state where relative sliding velocity exists at every point along the wrapped or "total arc" of contact between the belt and pulley. Hence, there is a shearing force due to sliding friction and a resulting change in belt tension along the total arc. PA0 2. "Creep" is defined as a state where there is (1) a first portion of the total arc where the belt sidewalls have zero velocity relative to the pulley and (2) a second portion of the total arc where the belt sidewalls have some velocity relative to the pulley. The arc having zero relative velocity, hence, no shearing force between the belt sidewalls and pulley due to sliding friction, is defined as "seating arc" and is characterized as a region of constant belt tension. The portion of the total arc having relative velocity is defined as the "active arc" and is characterized as a region of changing belt tension. There is relative velocity between the belt and pulley due to elongation or compression of the belt because of changing tension, or because of changing radial penetration of the belt in the pulley caused by changing tension.
It is well known that as the transmitted torque increases (Reference 6), a belt will change from the creep condition where there is both a seating arc and an active arc, to the slip condition where there is no seating arc. The slip condition limits the amount of torque that can be transmitted and is characterized by an ever increasing power loss.
The torque level at which slip impends can be empirically measured or calculated in accordance with Reference (3), supra, or as summarized in References (5) and (6).
Whatever the methodology for determining the onset of slip, it always occurs first on the driven pulley (Reference 6) because within the practical geometry of a variable speed drive the driver pulley will support a larger coefficient of traction than the driven pulley when slip impends. This is why the driven pulley is considered the critical pulley in determining the onset of slip.
Whatever the pulley actuation system for opening and closing the pulley halves, all variable speed drives must have sufficient belt tension to insure the existence of a seating arc to prevent slip for all required power loads.
The drives have actuators incorporating springs, fly weights, hydraulics or the like, to apply axial force and tension the belt. High tension may significantly reduce expected belt life by overstressing the belt tensile member. Some pulley actuators modulate driven pulley axial force, and hence belt tension, by means of a constant angle torque ramp and spring, to reduce the axial force as the pulley halves are separated. However, drives with such actuators are overtensioned because belt tension from the generated axial forces are substantially greater than the belt tension required to prevent slip.
Not only must a variable speed V-belt drive be properly tensioned, it must also be stable. As explained by Gerbert in Reference (4) supra, at page 9, a helical torque ramp in combination with a spring requires a strong-load spring and a weak torque-ramp action to produce a stable drive and avoid the condition where the driven pulley has a tendency to upshift with decreasing engine speeds. As an example, Gerbert uses a spring that is approximately 88 percent of the maximum axial force and a torque ramp action that has a maximum output of 12 percent of the total axial force when operating at a speed ratio of 1:1. Thus, the prior art V-belt transmission systems have driven pulley actuators which overtension the V-belt and impair belt load carrying life.